(1) Field of the Invention
The invention relates to a twin blade windshield wiper system for strong curved windshield surfaces. The invention is particularly useful to strong curved windshields, such as in rotary wing aircrafts like rotary wing aircrafts.
(2) Description of Related Art
The cleaning performance of wiper blades depends on a lean angle α between a wiper blade axis, which is perpendicular to the longitudinal extension of the wiper blade through the contact point of a wiper blade lip, and a perpendicular line to the windshield surface. The lean angle α can be defined by α=αT−(αC±ε). The angle αT is the target lean angle, which describes the necessary angle of rotation of the wiper blade axis perpendicular to the windshield surface. The angle αC is the wiper system configuration lean angle, which describes the angle of rotation of the wiper blade axis about the wiper blade longitudinal extension given by the control parts and/or piloting actuators of wiper system. The angle ε is the error tolerance lean angle, which comes for example from the low stiffness of the wiper system components or from other technical/physical aspects.
The best wiper cleaning performance of the wiper blade is provided when the wiper blade axis remains perpendicular to the windshield surface (α=0) while being driven between an end position (C) on a right side of the windshield, a central park position (A) and an end position (B) on the left side of the windshield, each of said positions (B) and (C) being angularly separated from the central park position (A) by an angle γ. Due to the strong curvature of modern windshields, particularly the windshields of rotary wing aircrafts, wipe cleaning performances are not continuous in all windshields areas. Especially towards outer left and right side positions of the wiper blade on the windshield, the angle α tends to deteriorate to higher angles α and therefore the cleaning performance of the wiper blade may become poor. Lean angles α higher than twenty degree leads to very poor cleaning performance and moreover the commonly metallic wiper blade frame touches the windshield and subsequently the bias of the lip of wiper blade against the windshield is restricted. A further consequence of the metallic wiper blade frame touching the windshield is that the frame may scratch the windshield during the wiper system operation and scratches in the windshield may deteriorate visibility for any crew behind the windshield.
The documents WO2005095170, FR2757815, FR2746355, U.S. Pat. No. 5,502,866 or WO9205982, US2002/0056168, EP0296081 and EP0351528 show wiper systems with control of the wiper blade lean angle α. An additional control arm or other mechanic control parts and/or piloting actuators for orienting the wiper blade perpendicular to the vehicle windshield are disclosed. Different windshields with different degrees of curvature necessitate different special control parts and/or piloting actuators according to said prior art resulting in a high number of different special control parts and/or piloting actuators respectively for small series at relatively high costs.
Another disadvantage of this prior art is the possible unstable wiper system behavior. This instability results from the forces exerted on the wiper blade. A spring force ensures the contact between the wiper blade lip and the windshield. A drive force causes the radial motion of the wiper blade on the windshield. Other forces exerted on the wiper blade are the aerodynamic, frictional and reaction forces. Different forces generate two different kinds of moments on the pivot-mounted wiper blade, i.e. the stable and the instable moment. Stable behavior of the wiper system is ensured, when the stable moment is equal to or greater than the instable moment. If the behavior of the wiper system is not stable, the wiper blade tilts over the windshield and the cleaning performance of the wiper system decreases. Further cases of the system collapsing are caused by the blockage of the guide wheel, e. g. sand in wheel or snow on the windshield or by strong curved windshield.
The document US2002/0056168 proposes a wiper system, able to orient the wiper blade perpendicular to the windshield, where the control of a local lean angle along the wiper blade (by the twist of wiper blade) is the target of the system. The additional orientation control of the local angle along the blade is more expensive, heavier and more complex in realization.
The document U.S. Pat. No. 6,272,717 describes a windshield wiper system supporting a pair of wiper elements via a wiper drive arm pivotably mounted proximate a vehicle windshield. A drive arm attachment member is adapted to be coupled with the wiper drive arm. A pivot arm is pivotably secured to the drive arm attachment member about a pivot axis which intersects the windshield. The pivot arm carries the pair of wiper elements such that each of the wiper elements moves over a corresponding first arcuate path when the drive arm is rotated in a first direction. Each paths of a first wiper element is different from the paths of a second wiper element. The wiper elements can also move between a upper position in a first direction, and a lower position when rotated in the opposite direction. Alternatively, each of the wiper elements moves between a first angular position and a second angular position in the opposite direction.
The document FR2878802 describes a glass wiper has a squeegee rotating around a longitudinal axis following a determined movement when radius of curvature of a windshield varies. The squeegee rotates such that a wiping blade supports against the windshield while preserving an angle between average angle and 90 degrees relative to the windshield. A drive arm driving the squeegee exerts a force in a mediator plane located between a blade and the wiping blade.
The document FR2490565 describes a wiping device for cleaning windscreens of motor vehicles. A central plane of the wiping strip remains constantly perpendicular with respect to the wiping point on the windscreen. A drive arm is connected to a drive shaft and a wiping element is connected via a cylindrical joint to the drive arm. The wiping element has identically shaped guide element. The two central planes run parallel and stand perpendicularly on the windscreen to be cleaned.
The document U.S. Pat. No. 4,553,283 describes a two-bladed windshield wiper adapter which tilts the wiper blades in the sweep movement direction. The adapter is formed with a plug receiving portion loosely fitted to a cross plate having two plug members disposed perpendicular thereto for receiving the wiper blades. The adapter has the ability to pivot and tilt at the position where the plug receiving portion is coupled to the cross plate.